The present invention relates to electric motor powered dental handpieces. More particularly, the present invention relates to an electric motor powered dental handpiece that utilizes an electric motor to directly drive a spindle chucking assembly that holds a desired tool.
Electric handpieces are currently being marketed. Electric motor driven dental handpieces are described in U.S. Pat. No. 4,278,429 to Straihammer et al.; U.S. Pat. No. 4,355,977 to Ota et al. and U.S. Pat. No. 4,486,176 to Tardieu et al. In some aspects, electric powered handpieces have advantages over air powered models, for example, electric powered handpieces exhibit superior speed regulation; provide an acceptable degree of speed regulation over a wide range of desired outputs speeds; and the torque that is supplied, particularly at lower speeds, is excellent. However, prior art electric powered handpieces have several disadvantages compared to air driven handpieces, including: increased weight; larger diameter and length; difficult or impossible to service in the dental office; lack of a fiber optic swivel; and price.
More specifically, currently marketed electric handpiece systems and their associated motors are approximately three times the weight of air handpieces. A significant amount of the additional weight is concentrated at the rear end of the electric handpiece handle. In some designs, the motor is physically located in the dental hose connection area which attaches to the rear portion of the handpiece handle. Most of the weight is due to the electric motor. The electric motor length of currently marketed electric handpiece systems causes the electric handpiece and attached hose connection combined length to be about 40% longer than the air handpiece length. Similarly, the motor diameter causes the electric handpiece diameter (in the area where the motor is located) to be about 30% larger than the maximum diameter of an air handpiece.
The additional weight of the electric motor and its location, significantly away from the head (where the cutting tool is chucked), combine to make electric handpieces awkward and tiring to use. The center of gravity of these electric handpieces are located beyond the normal gripping range of the dentist's hand.
A dentist basically grips a dental handpiece as one would grip a pencil while writing. The gripping range of a dentist's hand is comprised of two different types of grips separated by a distance. The gripping range begins with a combined “three finger grip” placed at the front of a handpiece. The area gripped is the lower handle portion of the handpiece and is located very closely to the head of the handpiece. The head contains the spindle assembly which is designed to rotate at a broad range of speeds. Variously shaped cutting tools can be mounted or “chucked” in the spindle to perform a variety of cutting procedures. The tips of the thumb, index finger and middle finger are oriented to surround the front portion of the handle in order to precisely locate the cutting tool. The gripping range ends with a “cradle” type of grip generated by the “crook” area of the dentist's hand that is placed under the rear portion of the handpiece handle. The physical distance between the front and rear grips is referred to herein as the “gripping span.”
FIG. 1 illustrates a force distribution diagram of a typical air powered handpiece 30 when gripped by a simulated dentist's hand 27. The air handpiece 30 generally consists of a head 31 which is fastened to a handle comprised of a lower handle portion 32 and an upper handle portion 33. The head 31 contains an air turbine (not shown) which rotates a cutting tool 34 at a high speed. The center of gravity 35 represents the approximate location at which the total weight “WA” of air handpiece 30 can be considered to be concentrated for weight distribution analysis. Reference numbers 20, 26 and 28 represent simulated segments of a dentist's hand 27. The lower handle portion 32 is primarily supported at front gripping area 36 by the dentist's middle finger 20. The upper handle portion 33 is supported at rear gripping area 37 by the crook area 26 of the dentist's hand 27. Gripping span 28 indicates the relative anatomical distance between middle finger 20 and crook 26 of the dentist's hand 27.
A typical air handpiece 30 weighs about 50 grams. When the air handpiece 30 is held by a dentist, its center of gravity 35 occurs within the dentist's gripping span. The actual location of the center of gravity 35 occurs at an upper handle portion 33 location which is approximately twice as far from the three finger grip at the front gripping area 36 as it is from the crook area 26 at the rear gripping area 37. The weight distribution is therefore about ⅓ (FAF≈WA/3) at the front gripping area 36 and ⅔ (FAR≈2WA/3) at the rear griping area 37. This computes to approximately 17 grams at the front gripping area 36 and 33 grams at the rear gripping area 37. A closer analysis of the weight distribution at the front gripping area 36 reveals that the 17 gram weight is virtually completely supported by the side of the middle finger 20. The thumb (not shown) and index finger (not shown) are used mainly to provide a very light lateral stabilizing force during actual cutting procedures.
The direction of the forces FAF and FAR created by the weight of the air handpiece 30 at the front gripping area 36 and rear gripping area 37, respectively, is downward in both cases. Having handpiece forces FAF and FAR oriented in a downward direction at the front and rear gripping areas of the dentist's hand 27 may not appear to be a significant advantage, however this configuration, which results from an air handpiece 30 having a center of gravity 35 located between the front gripping area 36 and rear gripping area 37, is desirable because it allows the front grip to be made with substantially less effort, compared to that required by the currently marketed electric powered handpieces, as will be described hereinafter.
FIG. 2 illustrates a force distribution diagram of a typical currently available electric handpiece 40 when gripped by a simulated dentist's hand 27. The electric handpiece 40 generally consists of a head 41 which is fastened to a handle 49 comprised of a lower handle portion 42 and an upper handle portion 43. The lower handle portion 42 and the upper handle portion 43 refer to general areas of the handle 49. Throughout the following text, in general, the lower handle portion 42 of our invention is considered to be the section of the handle 49 that is forward (toward the head 41) of the rear gripping area 47 of the handle 49. The upper handle portion 43 is considered to be the section of the handle that is rearward (away from the head 41) of the rear gripping area 47 of the handle 49. The handle 49 does not have to be comprised of two physical sections. It could be comprised of a single continuous piece of material or could be comprised of three or more sections of material. If the handle is constructed of two pieces, the mating plane 51 can occur at any place in the handle 49 assembly and not necessarily as illustrated in FIG. 3.
The head 41 contains a spindle chucking assembly (not shown) which rotates a cutting tool 44. The center of gravity 45 represents the approximate location at which the total weight “We” of electric handpiece 40 can be considered to be concentrated for weight distribution analysis. Reference numbers 22, 24, 26 and 28 represent simulated segments of the dentist's hand 27. The lower handle portion 42 is primarily gripped at front gripping area 46 by index finger 22 and thumb 24. The upper handle portion 43 is supported at the rear gripping area 47 by crook area 26 of a dentist's hand 27. Gripping span 28 simulates the relative anatomical distance between front gripping area 46, which is gripped by index finger 22 and thumb 24, and the crook area 26.
The threefold weight factor of electric versus air models and the weight distribution of typical current electric handpieces 40 require substantially more gripping effort to be supplied by the dentist. The center of gravity 45 of a typical electric handpiece 40 is located significantly farther from the head 41. The center of gravity 45 of electric handpiece 40 lies outside the typical hand gripping range described above. It lies about ⅓ of the “gripping span” distance beyond the crook area 26. A force distribution of an electric handpiece 40, assuming a weight of about 150 grams, results in a 50 gram force FeF at the front gripping area 46 and a 200 gram force FeR at the rear gripping area 47. The force FeF required to support the electric handpiece 40 at the front gripping area 46 is approximately three times what was necessary to support the air handpiece 30 at its front gripping area 36 (FeF≈WA≈3FAF). Additionally, the front gripping force FeF must be supplied in a downward direction for the electric handpiece 40. The rear gripping force FeR required to support electric handpiece 40 at the rear gripping area 47 or crook 26 area, is approximately six times that required to support air handpiece 30 at the same location (FeR≈4WA≈6FAR).
The downward direction and magnitude of the front gripping force FeF required to support electric handpiece 40 requires significantly more effort by the dentist. The thumb 24 and index finger 22 are still required to provide lateral stabilizing force during actual cutting procedures. However, in addition to providing lateral stabilizing forces, the thumb 24 and index finger 22 must also provide a downward force FeF. This force of about 50 grams is approximately three times as much as the upward force exerted by the middle finger 20 for an air handpiece 30. The middle finger (not shown on FIG. 2) is only very lightly used to support electric handpiece 40.
The approximate six fold increase in force FeR required at the rear gripping area 47 by the crook 26 area to support electric handpiece 40 compared to air handpiece 30, is one of the reasons why currently marketed electric handpieces are not particularly favored by dentists.
There is a related problem caused by the weight distribution of electric handpieces. Frequently a dentist will be gripping a handpiece, but he will not be using it to remove tooth material. For example, the dentist will not be using a handpiece to remove tooth material when he needs to view the progress of his work, to reposition the cutting tool or to install a new cutting tool. During these periods, the dentist will typically relax his grip on the handpiece to minimize fatigue. Because the air handpiece 30 is significantly lighter and because the air handpiece force is directed downward for both the front gripping area 36 and rear gripping area 37, the dentist can relax his grip significantly without any serious consequences. The thumb and index grips can almost be completely eliminated without any problems. However if the same type of relaxed grip is attempted with the front grip of electric handpiece 40, the front of electric handpiece 40 will rise and the electric handpiece 40 will slip out of the dentist's hand 27. Because the dentist's hand is usually very wet, it is possible for an accidental or unplanned reduction of front gripping force to occur during a cutting procedure. This could possibly result in the revolving cutting tool 44 damaging an otherwise healthy tooth surface. Also, assuming all other contributing factors are equal, the electric handpiece 40, which is three times the weight of an air handpiece 30, will be more likely to slip in the dentist's grip due to the increased weight.
Another disadvantage of current designs relates to serviceability. Air and electric handpieces have spindle drive components located in the head area that will need to be serviced at some point in time. This includes the two spindle shaft bearings and the spindle tool chucking assembly. Air handpieces generally have a removable threaded end cap located on the top of the handpiece head. Removal of this end cap allows the spindle chucking assembly, generally referred to as a “turbine cartridge” for air handpieces, to be easily removed for servicing. Replacement of the turbine cartridge of an air handpiece takes only two minutes or less and can be easily performed in the dental office. If an air handpiece turbine cartridge suddenly stops rotating in the middle of a procedure, it is practical to replace the cartridge while the patient remains in the dental chair.
Currently marketed electric handpieces employ spindle chucking assemblies that cannot be serviced by simply removing a top end cap. The complete spindle assembly cannot be removed because the motor gear on the motor shaft traps the lower spindle bearing. Cutting tool speeds in the vicinity of 220,000 RPM are required for a significant portion of dental procedures. Because electric motors having speeds greater than 110,000 RPM are not currently available, a 2:1 ratio speed increasing gear set is required to obtain cutting tool speeds near 220,000 RPM. Air and electric dental handpieces use miniature spindle bearings to keep the head diameter of the handpiece within acceptable limits. The bearings have limited life and will need to be replaced periodically.
The spindle chucking assembly of currently marketed electric powered handpieces includes a spindle gear which is driven by a gear located on the electric motor shaft. The spindle gear is mounted on the collet shaft of the spindle chucking assembly and is located between the two spindle bearings. In order for the lower spindle bearing to clear the motor gear on removal (as part of the spindle chucking assembly), the spindle gear would need to have an outside diameter greater than the lower spindle bearing. This would require the motor gear, because of the 2:1 drive ratio, to have an outside diameter approximately twice that of the spindle bearings. A motor gear having a diameter approximately twice the diameter of the spindle bearings is not practical because the head and lower handle portion diameters adjacent to the head would need to be enlarged significantly to clear the motor gear. The resulting lower handle portion diameter would be too large for the dentist to comfortably grip.
Currently marketed electric handpieces require the entire motor assembly and all jackshaft assemblies be removed in order to remove the lower spindle bearing. Unfortunately, removal of the electric motor assembly and associated jackshaft assemblies cannot be initiated until other electric handpiece components are removed. The other electric handpiece components include the upper and lower handle portions, the four fluid carrying tubes and the fiber optic light pipe. Additionally, electrical contacts and associated lead wires to the motor windings would also have to be removed. This is a very involved procedure which is impractical to perform in the dental office. As a result, currently marketed electric powered handpieces typically need to be sent to a dealer or manufacturer for servicing. Sending an electric handpiece to a dealer or manufacturer for servicing is a major problem because it requires the dentist to spend additional money to purchase a spare electric handpiece for use whenever his original electric handpiece needs to be serviced.
Furthermore, current air and electric handpieces typically use multiple nozzles to deliver an air and water spray mixture to cool the cutting tool and tooth as well as to wash away cutting debris. A multiple nozzle spray configuration is described in U.S. Pat. No. 3,199,196 to Lieb et al. The nozzles are equally spaced around the lower head periphery of the handpiece and are designed to produce spray jets from multiple directions. A multiple spray nozzle arrangement of at least three nozzles is advantageous because frequently a portion of a tooth will deflect one or two of the multiple spray jets away from the cutting tool work zone on a tooth. A minimum of three spray nozzles insures that there will always be at least one undeflected spray jet to cool the cutting area. Thus cutter tool and tooth overheating will be avoided. Single jet spray handpieces will need additional spray provided by a dental syringe in cases where a portion of a tooth deflects the spray jet. Although employment of a syringe to provide additional air and water spray is sufficient to keep the cutting zone cool, it is undesirable in that it adds yet another tool to the operating zone in the patient's oral cavity.
Multiple spray nozzle configurations generally involve the use of two circular distribution chambers for air and water. Each chamber is supplied air or water via a single tube inlet. Unfortunately, over time and particularly with dental operatories having relatively hard supply water, scale deposits will grow in the water spray lines. At some point, the resulting spray will become inadequate and the flow restriction will need to be removed. Approximately 70% of the water flow path in typical multiple spray nozzle configurations can be cleaned with a small diameter wire or a miniature drill bit mounted in a pin vise. The remaining 30% of the situations that cannot be cleaned will require the handpiece be sent back to the manufacturer. The manufacturer will then have to machine away a portion of the spray chamber wall to gain access to the restriction. Following removal of the restriction a new chamber wall will then need to be pressed into place. Again, sending of the handpiece to a dealer or manufacturer for servicing is not desirable.
Another reason that discourages dentists from purchasing currently marketed electric powered handpieces is associated with an optional fiber optic illumination provision and an optional swivel connection. A significant percentage of air handpieces purchased by dentists incorporate either or both of these options. Fiber optic illumination is used to provide additional lighting to the cutting tool work area. A swivel connection between the handpiece and delivery hose substantially decreases the reactionary drag torque caused by the delivery hose.
Depending on the area of a tooth to be prepared and surrounding clearances, it may be necessary to rotate the handpiece about the long handle axis to position the cutting tool at an optimum angle. If a swivel connector is not employed, the section of the delivery hose immediately attached to the rear handle area of the handpiece will need to rotate through the same angle as the handpiece. Because the opposite end of the delivery hose is attached to the dental delivery system, the opposite end does not rotate. Therefore, whenever the handpiece is rotated as described above, the delivery hose is subjected to a net “twisting” displacement. A twisting torque must be supplied to the handpiece end of delivery hose to cause the net twisting.
The amount of reactionary drag torque necessary to rotate the handpiece end of the delivery hose is directly proportional to the relative amount of twisting displacement of the delivery hose caused by the handpiece rotation. A torque must be supplied along the handpiece handle to counteract the delivery hose drag torque. The torque is supplied by the dentist's three finger grip at the lower handle portion of the handpiece. Application of the torque requires the dentist to supply a circumferential force at each of the three fingers involved in the grip. Also, a radially inward gripping force must be supplied by the three fingers to insure the handpiece handle does not rotationally slip relative to the dentist's hand. The circumferential and radial forces required to counteract the delivery hose torque are in addition to the gripping forces necessary to support the weight of the handpiece described earlier.
A dual fiber optic and swivel option is available with most air handpieces. In such dual option air handpieces, the fiber optic light pipe is typically placed at the central axis of the swivel connector in order to keep the design as simple as possible. However, currently available electric handpieces which have swivel connections typically locate the motor shaft, or one of the two jackshafts, at the central axis of the swivel connector to keep the mechanical design as simple as possible. As such, it is not possible to have the rotating shaft and the fiber optic light pipe sharing the same central axis.
An additional factor responsible for the relatively low sales of current electric powered handpieces is price. Current electric handpieces cost more than air handpieces because of the need to provide electronic controls and the costs of the electric motor and the two jackshaft subassemblies.
Despite the control disadvantages of air powered handpieces and the significant control advantages of electric powered handpieces, the fact that only approximately 15% of handpieces sold in the United States are electric, indicates that dentists consider these disadvantages of prior art electric handpieces to be very significant.